A probe microphone must be able to measure the sound pressure at a point for instance in a very hot environment. An oblong probe tube in connection with a microphone cartridge, gives, however, some unwanted resonances. It has been attempted to solve this problem by means of an almost infinitely long tube to which a branch tube is connected, the branch tube being connected to a cavity and a microphone cartridge. As a result, unwanted resonances in a portion of the frequency interval are reduced. However the microphone cartridge and the associated attachment is an unwanted load, especially at high frequencies.